The present invention pertains to continuous reactor processes and in particular the use of such reactors to effect reaction between a liquid and a reactant gas.
In the manufacture of precipitated calcium carbonate it is conventional to use a batch reactor, a continuously stirred tank reactor (CSTR) or a pipe-line-type plug flow (PF) reactor to contact a liquid slurry of water and calcium hydroxide with carbon dioxide in order to synthesize precipitated calcium carbonate having particular characteristics.
Continuous stirred tank reactors rely upon a mechanical agitator and the introduction of the reactant gas directly into the liquid to achieve the desired reaction. The continuous stirred tank reactor is operated at predetermined temperatures, pressures and agitation rates in accord with the product being produced by the contact of a liquid with a reactant gas. Continuous stirred tank reactors are generally limited in size. In order to achieve increased system throughput or economics of scale, multiple reactors must be employed.
The plug flow reactor is generally a long tubular shape reactor filled with the liquid which is generally moving in a straight line direction into which the reactant gas is introduced. Plug flow reactors are generally expensive since they require a long pipe line and the use of a high purity gases in certain applications. Two reasons for using high purity gas are, to avoid slugging and to enable the use of smaller size pipe.
Numerous techniques have been used to produce precipitated calcium carbonate having a controlled particle size for use in various applications and in particular the treatment of papers.
U.S. Pat. No. 2,538,802 discloses and claims a continuous process for producing precipitated calcium carbonate having a desired particle size range using a two-stage dual carbonator system. [Patentees give details of other reactors that were available at the time, i.e. prior to 1951.]
U.S. Pat. No. 3,150,926 discloses and claims a continuous process for producing precipitated calcium carbonate using an elongated reactor having dual screw type conveyors to move the slurry from the entry end to a discharge end of the reactor. Paddles and longitudinal blades are used to move the materials through the reactor in what patentees describe as a flow pattern “likened it to a rock and curve-bound stream wherein the stream flow is basically in one direction although the obstacles and curves create back flows, eddys and swirls which slow the rate of flow while keeping the entire stream in a constant state of agitation.” Patentees also described the action as that of a “mechanically fluidized bed.” The reactor is enclosed and carbon dioxide is introduced through the bottom of the reactor in what is called the carbonation zone.
U.S. Pat. No. 4,133,894 discloses and claims a multi-step, multi-vessel process for preparing precipitated calcium carbonate having less that a 0.1 μm particle size. Various processing parameters are disclosed.
U.S. Pat. No. 4,888,160 discloses and claims using a stirred tank reactor for preparing various precipitated calcium carbonate products. The Patent discloses control of various parameters, e.g. pH, composition of the slurry, temperature, reacting gas purity, and the use of inhibitors to achieve the desired particle shape.
Other types of reactors which show varying types of flow to introduce a gaseous reactant into a slurry are exemplified by U.S. Pat. Nos. 2,000,953; 2,704,206; 3,045,984; 3,417,967; 3,856,270; 4,313,680; and 4,514,095. All of the foregoing reactors use complex mechanisms to provide a motion or direction change to a slurry moving through the reactor to enhance gas-liquid contact.
There is a need to provide for both improved processes for gas liquid contacting and improved apparatus that can be fabricated easily and economically to carry out such processes.